FIG. 4 illustrates an example of a conventional coal-fired boiler for power generation and an exhaust gas treatment apparatus thereof.
Coal supplied from a coal supply line 21 is burned in a boiler 1 by air supplied from a combustion air supply line 36, and the generated high-temperature exhaust gas is heat exchanged by a heat exchanger 11 provided at an outlet of the boiler 1 to generate steam, such that a power generator 13 is operated with the steam supplied by a steam turbine 12. The low-pressure steam discharged from the steam turbine 12 is gradually cooled by a condenser 14, then is again pressurized by a pump 15 and sent to the heat exchanger 11.
The combustion exhaust gas is treated as follows. It is configured in such a way that: first, nitrogen oxide (hereinafter, referred to as NOx) in the exhaust gas is reduced to nitrogen by a denitration catalyst in a denitration device 2 installed on an exhaust gas flow downstream side of the boiler 1, then a gas temperature is decreased by an air heater (A/H) 3, and ash is removed by a dust collector 4; and next, sulfur oxide (hereinafter, referred to as SOx) is removed by a desulfurization device 5, then is discharged from a chimney 16 to the atmosphere, thereby removing ash dust, NOx, and SOx in the exhaust gas.
In the desulfurization device 5, a desulfurization absorption liquid containing an absorbent such as a slurry containing limestone (calcium carbonate) or lime is sprayed as fine droplets from desulfurization spray nozzles 27. The droplets of the desulfurization absorption liquid and the exhaust gas contact with each other, such that the SOx in the exhaust gas is chemically absorbed and removed by surfaces of the absorption liquid droplets from the desulfurization spray nozzles 27, together with an acidic gas such as ash dust, hydrogen chloride (HCl), and hydrogen fluoride (HF) in the exhaust gas.
The absorption liquid absorbing the SOx (mainly SO2) is once collected in a circulation tank 28 at a bottom of the desulfurization device 5, and is oxidized by the air supplied from an air supply pipe for oxidation (not illustrated) to generate calcium sulfate (gypsum). A part of the absorption liquid extracted from the circulation tank 28 is supplied to the desulfurization spray nozzles 27 through the circulation pipe 25 by a pump 26, and the remaining part thereof is separated from the gypsum by a gypsum separator 29, and then gypsum 30 is recovered.
The absorption liquid separated from the gypsum 30 is returned from a storage tank 31 to the desulfurization device 5 through a desulfurization absorption liquid return pipe 35 by a pump 32, or a part thereof is sent to a wastewater treatment device 67. In addition, a part thereof is returned to a calcium carbonate supply device 44, and is also used to adjust the absorbent such as a limestone slurry. In the wastewater treatment device 67, harmful substances including heavy metals contained in the wastewater are removed, then a waste fluid is discharged to an outside the system.
In recent years, regulations against the mercury contained in the coal have been increasingly reinforced, and several methods for removing heavy metals, mainly the mercury from the exhaust gas have been proposed and practically used.
According to Patent Document 1 below, by reinforcing a mercury oxidative function of the denitration catalyst, a mercury removal characteristic is improved. The mercury in the coal is discharged into the exhaust gas as a form of elemental mercury in a combustion field. This elemental mercury is minimally adsorbed to a solid such as ash, and also has low water solubility. In this form, the elemental mercury passes through an exhaust gas treatment apparatus such as a conventional dust collector or desulfurization device, and is discharged to the atmosphere. Therefore, in Patent Document 1, the mercury oxidative function that converts the elemental mercury into oxidized mercury is added to the denitration catalyst. The oxidized mercury is easily adsorbed to the solid, and has high water solubility. A part thereof is attached to the ash in the exhaust gas and is removed together with the ash by the dust collector, and most thereof is absorbed into the absorption liquid of the desulfurization device, such that releasing to the atmosphere may be prevented.
In addition, according to Patent Document 2 below, by adding a halogen compound to an exhaust gas flow upstream side of the denitration catalyst, the mercury removal characteristic is improved. Mercury is reacted with halogens (chlorine and bromine) in the exhaust gas to become oxidized mercury. If an amount of these halogens in the coal is small, a ratio of mercury converted into the oxidized mercury is reduced. Therefore, in Patent Document 2, by adding the halogens to the exhaust gas flow upstream side of the denitration catalyst, the ratio of mercury in the exhaust gas converted into the oxidized mercury is increased, thereby increasing a mercury removal rate in the exhaust gas.
Further, according to Patent Document 3 below, by adding a mercury adsorbent such as activated carbon to a duct on the exhaust gas flow upstream side of the dust collector, mercury in the exhaust gas is adsorbed by the adsorbent, and is recovered and removed by the dust collector together with the ash.
Furthermore, according to Patent Document 4 below, a scrubber is provided in an exhaust gas flow path between the dust collector and the desulfurization device, and activated carbon is added to the exhaust gas to adsorb the mercury and then washed by the scrubber. The waste fluid after washing is separated into a solid component and a liquid component by a solid-liquid separator, and the liquid component is returned to the scrubber to be reused. In addition, activated carbon of a solid component is dried at a temperature not exceeding 90° C. at which the mercury is not desorbed, and then is again added to the exhaust gas.